Cyclic carbonates are an important compound used in a wide range of applications as they are used as an organic solvent, a synthetic fiber processing agent, a pharmaceutical raw material, and a cosmetic additive, also recently used as an electrolyte solvent for lithium batteries, and further utilized in the synthesis of alkylene glycols or dialkyl carbonates.
This cyclic carbonate has been conventionally synthesized by reacting an epoxide with carbon dioxide in the presence of a homogeneous catalyst under a proper pressurized condition. As such a homogeneous catalyst, an onium salt such as a halide or a quaternary ammonium salt of an alkali metal, for example, has long been known (Patent Literature 1), and such a homogeneous catalyst is also industrially used.
However, the separation operation of the reaction mixture from the catalyst by, for example, distillation is usually required in the case of using such a homogeneous catalyst, and thus not only the producing process is complicated but there is also a problem such as the decomposition of the catalyst during the separation process or the generation of by-products.
Accordingly, a heterogeneous catalyst in which a quaternary phosphonium group having a halide ion as a counter ion is immobilized on a support such as silica gel has been proposed for the purpose of simplifying the catalyst separation process, and a method for continuously producing propylene carbonate by mixing propylene oxide with supercritical carbon dioxide and supplying the mixture to the reaction tube filled with the immobilized catalyst is disclosed as a method for producing propylene carbonate using the immobilized catalyst (Patent Literature 2).
However, the immobilized catalyst exhibits lower activity as compared to a homogeneous catalyst, and thus it is required to be used in a great amount and it is a problem to increase the reactor in size particularly in the case of producing a cyclic carbonate on an industrial scale.
In addition, the passing amount of the reaction solution with respect to the catalyst amount is smaller, and thus there is a problem that (1) the uneven flow of the reaction solution occurs in the reactor and (2) the contact of the catalyst with the reaction solution, namely, wetting of the catalyst becomes insufficient and thus the catalyst is not able to sufficiently function. Furthermore, for example, the uneven flow in the system causes a factor of hot spots (local overheating of the catalyst) and the deterioration of catalyst is significantly accelerated.
On the other hand, the uneven flow is generated in the system or wetting of the catalyst is insufficient, and as a result, a decrease in catalytic efficiency and catalyst lifetime is likewise caused in the same manner as the above, in a case that carbon dioxide is gasified in the reactor.
In addition, the reaction solution does not necessarily form a homogeneous phase when carbon dioxide is insufficiently mixed even under the condition in which carbon dioxide is not gasified. For example, propylene oxide and supercritical carbon dioxide are used by being mixed in Patent Literature 2, and the phase separation between propylene carbonate of the product and supercritical carbon dioxide is caused as described in Non Patent Literature 1. Hence, carbon dioxide is required to be completely mixed with the reaction solution in order to sufficiently dissolve carbon dioxide in the reaction solution and to suppress the phase separation in the reactor, and thus a large-scale ancillary facility such as a stirring tank is required.
Furthermore, when the temperature increases, the catalyst component is desorbed from the immobilized catalyst and the activity of the immobilized catalyst significantly decreases, whereas the reaction of an epoxides with carbon dioxide is an exothermic reaction to release a relatively great quantity of reaction heat (for example, the reaction heat released by the reaction of ethylene oxide with carbon dioxide is about 100 kJ/mol), and thus the removal of reaction heat at the time of the synthesis of a cyclic carbonate is a problem in the case of using an immobilized catalyst.
As the method for removing the react ion heat, it is a general method to use a heat exchanger-type reactor such as a jacketed reactor or a multi-tubular reactor.
However, the heat removal by a jacketed reactor to circulate the heat medium to the jacket has a basic problem that the heat removal area decreases as compared to the amount of catalyst when the reactor is increased in size and only the heat from the immobilized catalyst in the vicinity of the heat removal surface can be removed.
On the other hand, in the case of a multi-tubular reactor having a plurality of reaction tubes provided in the reactor shell, the reaction heat generated is removed by circulating the heat medium in the reaction tube shell while the reaction is conducted by filling the catalyst in the reaction tube, and thus it is possible to increase the heat removal area. However, in the case of using a catalyst immobilized on a support such as silica gel, a significantly little liquid flows as compared to this amount of catalyst flows, thus the reaction tube is required to be significantly thin and long in order to obtain a sufficient heat removal efficiency, and the apparatus is complicated and increased in size. In addition, the maintenance is also troublesome. Furthermore, there is also a problem that it is difficult to uniformly fill a plurality of reaction tubes with a catalyst.